1. Field of the Invention
The present invention relates to a zoom lens and more particularly to a zoom lens in which a focusing unit is composed of three sub units and the center sub unit among the three sub units is moved in the direction along the optical axis for focusing. The present invention also relates to an image pickup apparatus having such a zoom lens.
2. Description of the Related Art
There have been demands for reduction of the change in the angle of view with focusing in television cameras and video cameras. As a focusing method in a zoom lens that is preferred to this end, there has been developed a focusing method described below.
In this focusing method, a focus unit is arranged as the first unit, which is composed of three sub units, and the center sub unit among the three sub units is moved in the direction along the optical axis to effect focusing.
With reduction in the overall size of cameras, reduction in the size of lens systems (in particular, the size of front unit of lens systems) is also demanded.
Reference is made by way of example to Japanese Patent Application Laid-Open No. 2001-021804. A zoom lens disclosed in this patent document includes, in order from the object side to the image side, a first lens unit having positive refractive power, a second lens unit for zooming having negative refractive power, a third lens unit that corrects image plane variation with zooming, and a fourth lens unit having an imaging effect that is stationary during zooming. The first lens unit is composed of a first sub unit having negative refractive power that is stationary for focusing, a second sub unit that moves along the optical axis during focusing, and a third sub unit having a positive refractive power that is stationary for focusing. The zoom lens disclosed in this patent document is designed to have high optical performance (with improved distortion correction in particular in the wide angle range) by setting the focal lengths of the first to third sub units and the first lens unit appropriately.
In the prior art disclosed in Japanese Patent Application Laid-Open No. 2001-021804, high performance is achieved throughout the entire zoom range and the entire focus range generally by correcting chromatic aberration on a unit-by-unit basis in a zoom lens having a high zoom ratio equal to or higher than 10. For example, in the first embodiment, chromatic aberration is corrected in each of the first sub unit and the third sub unit independently from each other to reduce overall chromatic aberration of the first lens unit. However, correction of chromatic aberration made on a unit-by-unit basis leads to an increase in the number of lenses constituting the first lens unit, making the size of the first lens unit larger.